The Greater Good is a decent novel, though in my opinion the worst in the Ciaphas Cain series.
I should preface by saying that I am a massive fan of the Cain novels. I think that Mitchell has the best grasp of that mixture of comedy and horror/violence that is the hallmark of the 40k setting. His novels combine scathing wit with really solid action and adventure.
That being said, The Greater Good does follow this formula, but in my opinion, it feels like it is going through the paces. Yes, Cain makes disparaging remarks about his own achievements, to Amberley's chagrin. Yes, Cain makes mention at every opportunity that he was looking for a way to run away from danger. Yes, every time Jurgen walks into the room we are reminded that he smells horrible.
However, all of that feels like we've been there and done that before. The jokes seem tired and repeated, and the whole adventure feels without particular enthusiasm. It was as if the joke has run its course, and it was time to move on. If that's the case, I would be fine. The Cain series stands perfectly fine on its own. However, this slightly-disappointing sequel left a bittersweet taste in my mouth.
The promise of this novel, about the Tau and the Greater Good, goes nearly unfulfilled. There is one portion, early on in the novel, when you feel like Cain's going to be sent off on an epic adventure that will truly live up to the title's premise. Then, in about a paragraph, someone else is given the job and Cain's off on another generic, Imperial Guard (er... Astra Militarum?) shoot-em-up. The Tau could have been replaced by any sort of diplomat, snivelling politician, or aloof military official. The Xenos' involvement is minimal, and to be honest, unnecessary.
Perhaps they are setting up for a sequel. Perhaps this was all build-up to a new book, where they will play a bigger role. I can see that, but it doesn't make this one any less bitter to swallow.
However, The Greater Good is still an exciting and competent sci-fi military thriller. It has good action, likeable characters, high stakes, and a satisfying conclusion. It is by no means a poor book, or one you regret reading. It just doesn't live up to the magic of a blue-faced colonist shouting about their "FREEDOM!" or Cain knocking a Khornate Cultist out while shouting about his favourite football team.